1. Field of the Invention
The invention relates to a method of manufacturing an optical fiber, whereby a fiber is drawn from a molten extremity of a preform and is subsequently subjected to a torque, thereby causing a portion of the fiber to be twisted about its longitudinal axis and to be endowed with a spin. More particularly, the invention relates to an optical fiber of this type having low polarization mode dispersion.
2. Technical Background
Light traveling in an optical fiber has two polarization modes. For optical fibers that are perfectly circularly symmetric in both geometry and internal and applied stress, operation at a wavelength or in a wavelength range which is regarded as “single-moded” actually supports two orthogonal polarization modes, wherein the two polarization modes are degenerate, propagate with the same group velocity and have no time delay after traveling the same distance in the fiber. However, in practice, optical fibers are not perfectly circularly symmetric. For example, imperfections such as geometric and form deformation and stress asymmetry break the degeneracy of the two modes. See, for example, Rashleigh, S.C. , Journal of Lightwave Technology, LT-1:312-331, 1983. As a result, the two polarization modes propagate with different propagation constants β1 and β2. The difference between the propagation constants is termed birefringence δβ, the magnitude of the birefringence being given by the difference in the propagation constants of the two orthogonal modes:δβ=β1−β2  (1)
Birefringence causes the polarization state of light propagating in the fiber to evolve periodically along the length of the fiber. The distance required for the polarization to return to its original state is the fiber beat length LB, which is inversely proportional to the fiber birefringence. In particular, the beat length LB is given by:                               L          B                =                              2            ⁢            π                                δ            ⁢                                                   ⁢            β                                              (        2        )            
Accordingly, fibers with more birefringence have shorter beat lengths and vice versa. Commercial fibers exhibit a wide variety of beat lengths since the geometric and stress asymmetries of such fibers vary along the length of the fiber and between different fibers. Typical beat lengths observed in practice range from as short as 2-3 millimeters (a high birefringence fiber) to as long as 10-100 meters (a low birefringence fiber).
In addition to causing periodic changes in the polarization state of light traveling in a fiber, the presence of birefringence means that the two polarization modes travel at different group velocities, the difference increasing as the birefringence increases. The differential time delay between the two polarization modes is called polarization mode dispersion, or PMD. PMD causes signal distortion, and thus PMD is very detrimental in high bit rate systems and analog communication systems. For a uniform linear birefringent fiber without perturbation, i.e. externally imposed perturbation, the PMD of the fiber increases linearly as the fiber length increases. However, in a longer length, random mode coupling is inevitably introduced into the fiber due to externally imposed perturbations, and statistically the PMD increase along the fiber is thus proportional to the square-root of the fiber length.
A known method of combating PMD is to deliberately spin the warm fiber as it is drawn from the preform, so that a mechanical spin becomes “frozen” into the fiber as it cools. The resulting rotation of the birefringence axis in the fiber produces continual mode-coupling between the orthogonal polarization modes of a carried signal, thereby inhibiting the accumulation of a significant phase lag between the two modes, and consequently causing a significant reduction in the fiber's PMD.
A method as specified in the opening paragraph is known from U.S. Pat. No. 6,324,872, wherein the drawn fiber is caused to pass over a roller whose rotational axis can be canted, so that the pulley can be caused to rock back and forth about an axis perpendicular to its rotational axis. The rocking motion of the roller produces a twist in the fiber along a substantial portion of its length. In particular, portions of warm fiber which are twisted in this manner will become endowed with a permanent twist (spin) as their constituent material subsequently cools.
The cited document stipulates that the spin imparted to the fiber ideally has a non-constant spatial frequency. This can be achieved by canting the pulley back and forth in a non-periodic manner. In this way, the described method aims to achieve a PMD of less than 0.5 ps/km1/2.
However, the known methods for spinning optical fiber to reduce PMD have some deficiencies. For example, the quality with which optical fiber is being manufactured today is becoming increasingly improved. Consequently, even unspun fiber now has the capability of exhibiting a PMD less than 0.1 ps/km. Unfortunately, prior art spinning methods have not been completely successful in reducing this already low level of PMD which is existent in some of today's manufactured single mode fibers to even lower levels.